The static imbalance of data storage disc stack assemblies, usually expressed in milligram-centimetres (“mg-cm”), has recently become a critical performance parameter in disc drive design. This is primarily due to the increasing demand for precision performance in the consumer data storage product market. High imbalance of the disc stack may lead to structural vibration and undesirable noise, both of which are unacceptable in consumer products such as games-boxes and audio-video products. Furthermore, the industry standard specification for drive level imbalance has been reduced from an industry standard, 70 mg-cm to the current state of the art, 50 mg-cm.
The major contributors to the disc stack imbalance of a typical disc drive include disc clamp offset, clamp mass, clamp notch size, motor “MR”, and media “MR”, where MR is a function of the mass (M) of the media or motor in (mg) x the offset (R) of the media or motor in (cm) from the center of gravity (CG). Of these major contributors, research has shown that the most sensitive contributors that could impact process yield are the clamp offset and the angular orientation or direction of the clamp offset. Clamp offset is defined as the radial distance between the center of the clamp and the center of the spindle motor to which the clamp is attached.
The disc stack typically includes one or more data storage discs clamped onto a motor hub flange via a stamped metallic clamp. The clamp design may also include a spring expansion ring to self-center the clamp with respect to the motor axial centerline. The condition of the machine that is used to assemble the disc clamp onto the stack, the centering of the ring, as well as the clamp shape are all factors that can contribute to the overall stack imbalance. With an industry standard of 50 mg-cm for static imbalance, and with the incoming drive components being at optimum condition, current processes for the assembly of disc drives still have poor process capability. Some of the processes generate 2–9% of their disc drives at a static imbalance higher than the 50 mg-cm standard. These imbalances are often compensated for by adding weights or screws to the motor hub or by physically offsetting the data storage discs on the motor hub flange.
Research has shown that the disc clamp offset and the clamp offset angle are two of the most important factors contributing to static imbalance. However, corrective actions typically involve major design changes and added cost such as active balancing which requires a design change and the addition of counter-balance weights. These solutions also require extra manufacturing floor space and labor to perform the corrective balancing.
Accordingly there is a need to develop a way of compensating for the disc clamp offset without a major design change or extensive costs. The embodiments of the present invention provide a solution to this and other problems, and offers other advantages over the prior art